1. Field of the Invention
The present invention relates to an apparatus for supplying a polishing liquid used for polishing a surface of a semiconductor substrate, and more particularly to an apparatus for supplying a polishing liquid containing uniformly dispersed abrasive particles therein to a polishing surface of a polishing unit in a stable condition.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus having a polishing unit (or section).
Conventionally, a polishing unit has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth constituting a polishing surface is attached to the upper surface of the turntable. A semiconductor substrate to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive particles is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor substrate held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
FIG. 7 is a schematic view showing the essential parts in an example of a polishing unit. The polishing unit comprises a turntable 142 having an upper surface to which a polishing cloth 140 is attached, a top ring 144 for holding a semiconductor wafer W as a polishing object and pressing the semiconductor wafer W against the polishing cloth 140 while rotating the semiconductor wafer W, and a polishing liquid supply nozzle 146 for supplying a polishing liquid Q to the polishing cloth 140. The polishing cloth 140 constitutes a polishing surface. The top ring 144 is coupled to a top ring shaft 148, and is vertically movably supported by an air cylinder (not shown).
The top ring 144 has an elastic pad 150 made of polyurethane or the like on its lower surface, and the semiconductor wafer W is held in such a manner that the semiconductor wafer W is closely brought in contact with the elastic pad 150. The top ring 144 is further provided with a cylindrical guide ring 152 on its outer periphery so that the semiconductor wafer W is not dislodged from the lower surface of the top ring 144. The guide ring 152 is fixed to the top ring 144, and the lower end of the guide ring 152 projects from the holding surface of the top ring 144 so that the semiconductor wafer W is retained in a recess defined by the holding surface of the top ring 144 and the guide ring 152.
In the polishing unit having the above structure, the semiconductor wafer W is held by the lower surface of the elastic pad 150 of the top ring 144, and pressed against the polishing cloth 140 on the turntable 142 by the top ring 144, and the turntable 142 and the top ring 144 are rotated so as to cause a relative sliding motion between the polishing cloth 140 and the semiconductor wafer W. At this time, a polishing liquid Q is supplied from the polishing liquid supply nozzle 146 to the polishing cloth 140. The polishing liquid comprises abrasive particles such as silica particles suspended in a chemical solution such as an alkali solution, and the semiconductor wafer W is polished by a combination of chemical polishing with alkali and mechanical polishing with the abrasive particles.
In order to perform a high quality polishing in the above polishing unit, it is necessary to supply a polishing liquid having a constant concentration at a constant flow rate to the polishing surface of the polishing unit. A polishing liquid supply system may comprise a storage tank for storing a condensate comprising a mixture of, for example, alkali such as KOH or NH4OH and silica particles, an adjusting tank for adjusting the condensate to a desired concentration by diluting the condensate with a liquid such as pure water or chemical solution, a supply tank for storing temporarily a polishing liquid adjusted in the adjusting tank, and a polishing liquid supply pipe for delivering the polishing liquid to a nozzle from the supply tank. The polishing liquid supply system further comprises pipes for connecting the tanks.
It is known that a polishing rate and a polishing quality in a polishing process depend on the concentration of the polishing liquid used for polishing semiconductor wafers. On the other hand, for reducing equipment cost and operating cost, a common polishing liquid supply source is required to be used for a plurality of polishing units. For this reason, the polishing liquid having a given concentration is temporarily stored in the adjusting tank or the supply tank, and is then delivered to the respective polishing units. As a result, the quality of the polishing liquid stored in the adjusting tank or the supply tank is degraded with an elapse of time, and hence the abrasive particles tend to aggregate for thereby making the effective size of particles larger. Thus, an undesirable polishing liquid containing excessively large abrasive particles may be delivered to the polishing units to cause a polished surface of the semiconductor substrate to be scratched or to decrease a polishing rate.
It is therefore an object of the present invention to provide an apparatus for supplying a polishing liquid which can continuously supply a polishing liquid having stable properties to a polishing section to perform a high quality polishing at all times.
In order to achieve the above object, according to a first aspect of the present invention, there is provided an apparatus for supplying a polishing liquid to a polishing section, comprising: a supply tank for storing a polishing liquid having given properties; a supply pipe for supplying the polishing liquid to the polishing section; a sensing device for detecting properties of the polishing liquid flowing through the supply pipe; and a stabilization device for maintaining properties of the polishing liquid stored in the supply tank or flowing through the supply pipe within an allowable range on the basis of an output signal from the sensing device.
According to the present invention, the properties of the polishing liquid supplied to the polishing section are continuously monitored, and when the polishing liquid has been degraded up to a condition that scratches are likely to be formed on a polished surface of the substrate by aggregated abrasive particles, various remedial procedures can be taken to stabilize the properties of the polishing liquid. Thus, the polishing liquid having stable properties is continuously supplied to the polishing section to provide a high quality polishing at all times. If it is found that the polishing liquid has been degraded to a large degree, then the operation of the polishing apparatus may be stopped to prevent inferior products from being produced.
In a preferred embodiment, the sensing device may measure at least one of particle size distribution, the number of coarse particles, oxidation-reduction potential, and solid material concentration in the polishing liquid.
According to the present invention, degradation of abrasive particles in the polishing liquid may be detected in real time directly or indirectly.
In a preferred embodiment, the stabilization device may uniformize particle size distribution.
The stabilization device for stabilizing the properties of polishing liquid may comprise an application device of ultrasonic energy to the polishing liquid, a filter for removing coarse particles, or an adding device of chemicals for adjusting the oxidation-reduction potential. The polishing liquid having a poor quality may be discarded, if necessary, to stabilize the properties of the polishing liquid.
In a preferred embodiment, the stabilization device comprises at least one of a filter for removing coarse particles, an ultrasonic generating device for breaking up coarse particles by ultrasonic energy, an adding device for supplying at least one of additives and abrasive particles to maintain a volume ratio of additives to abrasive particles in the polishing liquid at a constant value.
According to a second aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a substrate, comprising: a turntable having a polishing surface; a top ring for holding a substrate and pressing the substrate against the polishing surface; and a polishing liquid supply unit for supplying a polishing liquid to the polishing surface, comprising: a supply tank for storing a polishing liquid having given properties; a supply pipe for supplying the polishing liquid to the polishing surface; a sensing device for detecting properties of the polishing liquid flowing through the supply pipe; and a stabilization device for maintaining properties of the polishing liquid stored in the supply tank or flowing through the supply pipe within an allowable range on the basis of an output signal from the sensing device.